This invention relates to compositions comprising alkyl phosphonium salts that are molten at low temperatures and to the use of such compositions as electrolytes in electrochemical cells.
A class of molten compositions which is of special interest in the field of electrochemical cells is the class of fused salt compositions which are molten at low temperature. These compositions can be useful as electrolytes. Such molten or fused salt compositions are mixtures of compounds which are liquid at temperatures below the individual melting points of the components. The mixtures can form molten compositions simultaneously upon contacting the components together or after heating and subsequent cooling.
One example of molten or fused salt compositions that are useful as electrolytes are the chloroaluminate salts discussed by Wilkes, J. S., et al; in J. Inorg. Chem., Vol. 21, 1263-1264, 1982. To form the molten salt, alkyl imidazolium or pyridinium salts are mixed with aluminum trichloride (AlCl.sub.3). Another example is the chlorogallate salts made from gallium trichloride and methylethylimidazolium chloride that are discussed in Wicelinski et al, "Low Temperature Chlorogallate Molten Salt Systems," J. Electrochemical Soc., Vol. 134, 262-263, 1987. The use of the fused salts of 1-alkylpyridinium chloride and aluminum trichloride as electrolytes is discussed in U.S. Pat. No. 4,122,245. Other patents which discuss the use of fused salts from aluminum trichloride and alkylimidazolium halides as electrolytes are U.S. Pat. Nos. 4,463,071; 4,463,072 and Great Britain Patent Application GB 2,150,74DA. Unfortunately, the alkylimidazolium salts are difficult to prepare, and the alkyl pyridinium salts are too easily reduced in an electrochemical cell system.
Another class of fused salt compositions are the compositions formed from a metal halide and a hydrocarbyl-saturated onium salt containing at least one aromatic group. These are disclosed in U.S. Pat. No. 4,764,440. In U.S. Pat. No. 4,839,249, low temperature molten compositions comprised of ternary alkyl sulfonium salts are disclosed. Unfortunately, the hydrocarbyl-saturated aromatic substituted onium salts have low conductivities. The sulfonium salts are difficult to prepare.
In view of the deficiencies of known compositions, other electrolyte compositions which have improved stability or conductivity or are more easily prepared are desired.